Investigation was carried out on the regulatory mechanisms generated in mixed lymphocyte reactions (MLR). Suppressor cells were found to be generated by in vivo or in vitro priming to alloantigen (or by in vitro culture in the absence of alloantigen). The suppressor cell populations generated in vivo and in vitro were compared in MLR and cell-mediated lympholysis (CML) for antigen specificity, radiation sensitivity, mitomycin sensitivity, T cell dependence and ability to suppress via a soluble factor. These comparisons were carried out to permit analysis of regulatory influences which may be involved in the generation of allograft responses.